F-P cavity is an important tool in optical and laser research and is applicable to an optical device such as an external-cavity semiconductor laser and a monolithic solid laser etc.
The retro-incident reflection light of the folded F-P cavity has a spectrum structure opposite to the F-P cavity and can provide optical feedback with narrow linewidth. Folded F-P cavities currently available are composed of discrete components. For example, FIG. 1 shows a structure, which is proposed by K. Döringshoff, I. Ernsting, R. H. Rinkleff, S. Schiller and A. Wicht. Such a folded F-P cavity (CAV) is composed of a coupler 101 and two reflectors 102 and 103, wherein the coupler 101 also serves as a folded reflector. The incident light enters the folded F-P cavity from the coupler 101. After reflection by reflectors 102 and 103 and coupler 101 in the cavity, two reflected beams are produced, that is, one reflected beam in the same direction and collinear with mirror-reflected light of the original incident light on the coupler 101, and one retro-incident reflection light beam in a direction opposite to and collinear with the incident light, wherein the latter can be output from the F-P cavity as light with additional function of frequency selection.
However, due to the difficulty of accurate tuning of discrete components, existing folded F-P cavities composed of discrete components are sensitive to outside inferences caused by sound, mechanical vibration and temperature vary. Further, such F-P cavity usually has a relatively large volume and poor system stability. Further, the confocal cavity in which the light path is folded once needs processing two spherical reflectors to form the resonance cavity, whereby increasing processing difficulties.